Fibrous composites are promising lightweight, high-strength materials which retain their strength at high temperatures. The fibers contribute high tensile strength to the refractory matrix and also impart resistance to crack propagation. The high toughness of such composites has been characteristically associated with limited bonding between the fibers and the matrix in order to give substantial fiber pull-out as a major source of toughening. On the other hand, brittle failure in unsuccessful composites and resultant flat fractures with essentially no fiber pull-out have been attributed to strong fiber-matrix bonding.
In addition to controlling fiber-matrix bonding, fiber coatings can provide chemical protection from attack by the matrix during processing, as well as limiting the occurrence over time of the oxidative embrittlement of refractory fiber composites. Moreover, fiber coatings can protect fibers from mechanical damage during handling and processing. Good toughness characteristics can be introduced into composites with refractory fibers and a refractory matrix only when there is very limited bonding between the fibers and matrix, and between adjacent fibers. However, many possible combinations of fiber and matrix result in strong bonding over most, if not all, of the range of practical processing conditions, thereby limiting the potential toughness ranges of these composites. Such a problem exists, for example, with oxide-based fibers, because they have a tendency to degrade or react with the matrix when the refractory materials are molded. The reaction between the fiber and the matrix results in a high degree of bonding which renders the toughening mechanism inoperable. This problem for oxide-based fibers can be solved by creating a barrier coating on the fibers. Preferably, a suitable barrier coating would be relatively inert, and would comprise a physical separation between the refractory matrix and the reinforcing fiber. The barrier would eliminate reactions between the oxide fiber and the refractory matrix or would substantially slow the kinetics of such reactions so that toughening would result in the composite.
On the other hand, the adhesion of oxide coating to the associated fiber substrate in composite applications must be satisfactory to withstand the stresses to which the coated fibers are subjected in forming the composite structure.
While there are many methods for creating oxide coatings on fibers, none is completely suitable. For example, chemical vapor deposition has been used to provide a variety of chemical coatings. Chemical vapor deposition is frequently unsatisfactory because it requires the careful and precise injection of predetermined amounts of reactive gases containing precursors of the desired coatings as well as precise control of the temperature. Only then can the gases react at or near the surface of the fiber and deposit the desired coating on the fiber surface. Control of the deposition thickness and the quality (e.g., uniformity) of the barrier coating is difficult. A serious drawback of chemical vapor deposition processing, in addition to the aforementioned mechanical difficulties, is the high cost of such processing.
Accordingly, new coating processes which could provide thin, substantially uniform chemical coatings on continuous multi-filament fiber tows, without requiring the use of expensive chemicals and/or processing steps would be desirable.